One of the goals of electronics packaging, in general, is to increase the capability of semiconductor devices so as to offer more speed, and programming features, while providing products having smaller, lighter physical sizes. For a given semiconductor technology, such as CMOS or gallium arsenide, this trend leads to higher power dissipation and also to higher heat, fluxes. Reliability of semiconductor devices and electronics packaging in general is coupled to operational temperatures, with lower temperatures promoting increased reliability. Therefore, in order to achieve greater operational capability from a semiconductor device without sacrificing reliability thereof achieving better thermal performance is essential. This trend has been observed since the inception of semiconductor devices in the industry, and is expected to continue for the foreseeable future.
Various methods for improving thermal dissipation of an electronic package have been introduced to accomplish this objective, examples being defined and illustrated in detail in the following U.S. Letters Patents and other pertinent documents:
U.S. Pat. No. 4,034,468-Koopman
U.S. Pat. No. 4,254,431-Babuka et al
U.S. Pat. No. 4,825,284-Soga et al
Jap. Pub. App. 3-77355(A)--Omura
U.S. Pat. No. 4,993,482-Dolbear et al
U.S. Pat. No. 5,088,007-Missele
U.S. Pat. No. 5,444,300-Miyauchi
Research Disclosure 340110 (8/92, No. 340)
U.S. Pat. No. 4,034,468 and 4,254,431 are assigned to the same assignee as the present invention.
Typically, electronics packages utilize a semiconductor device or devices. Such devices, also known as chips or die, generate heat during operation. The rate of heat generated is known as the power of the chip and, for a given semiconductor technology, is proportional to the speed and complexity of the chip.
Providing a thermally conductive path from the chip outward is one of the major challenges to electronics packaging technology. A thermal path must be provided which possesses as low a thermal resistance as possible, while satisfying stringent economic factors, assembly processing and handling constraints, and environmental considerations. As is known, the chip is electrically coupled to external circuitry of the package, which in turn may form a part of an overall larger structure, e.g., a microprocessor. Maintaining reliable connection in such assemblies is paramount. Further, the chip must be protected from damage, debris, and chemical attack by coating, protecting, overmolding, glob-top, encapsulating, or encasing the connected die with methods and materials well-known in the industry. Chips may be packaged in such a manner that the chip assembly may be subsequently attached to a circuitized substrate (a printed circuit board or a flexible circuit) which forms part of the aforementioned structure. Chips or electronic devices may also be electrically attached to a circuitized substrate using the well-known method of direct chip attach, the chip subsequently being encapsulated, encased, or otherwise protected with a quantity of protective material. Thus, the electronic device which dissipates power is electrically connected to a circuitized substrate by either direct attach or is connected as a packaged device.
In either case, heat must be dissipated from the device without interfering with the electrical connection to the circuitized substrate. It is known that some amount of heat can be removed from the device through the electrical connections and into the circuitized substrate. However, this heat must then be removed from the circuitized substrate and this arrangement may not provide the most thermally efficient path. It is well known that a thermally efficient path is that from the device directly to a nearby structure commonly known as a heatsink and subsequently to the external atmosphere surrounding the heatsink. Various heatsink designs (such as heatsink fin size, shape and spacings) and materials (e.g., aluminum) for optimal performance thereof are known in the art; however, attachment of the heatsink to the device often leaves much to be desired in a thermal sense.
Typically, heatsinks are adhesively bonded directly to the flee of a semiconductor device. This method of attachment utilizes a thermally efficient adhesive, typically a thermosetting epoxy, provided in a thin layer. The heatsink is typically attached to the device after the device has been electrically connected to the circuitized substrate, so that the heatsink does not interfere with that connection process (typically solder wave or solder reflow processes).
One key limitation of this method of attachment is that the thermal adhesive is not removable once it has set. Thus, the entire device must be removed from the circuit card after the heatsink is applied because the heatsink alone may not be removed. If component rework, nearby device changes, or other factors requiring specific thermal processing which cannot tolerate a heatsink present exist, the entire device must be scrapped, which is obviously economically undesirable.
In order to avoid this limitation, a separable connection between the device and the heatsink is desired. In the art, it is common and known to simply press a flat heatsink base onto a flat component face and hold it there with screws, springs, or other retention hardware which allows the heatsink to be removed as needed. However, in practice, this "dry interface" is thermally inefficient. Because of unavoidable tolerances in manufacture of both devices and heatsinks, neither are ever perfectly flat. Thus, the dry interface will have gaps between the device and heatsink surface, such gaps greatly reducing thermal transfer efficiency.
To improve the thermal transfer efficiency, it is also known to include a quantity of thermal-transfer enhancing material such as a thermal grease or conformable thermally conductive material. With thermal grease (one example being alumina-filled silicon grease), containment of the grease in the interface area is a design and manuring issue, as leakage can contaminate the circuit board and promote drying and loss of thermal efficiency of the grease interface. Use of thermally conductive conformable material, such as an alumina or aluminum-nitride filled silicon elastomer, is known, but this material has limited thermal efficiency (compared to a solid metal such as solder) and limited ability to fill gaps and conform to the heatsink and device faces without excessive pressure being applied. It is also noted that relatively large research and development expenses are involved in proper grease development and containment.
A further consideration is that with temperature change, an electronic device may change size in several ways. The device may simply expand, in which case whatever shape it started with remains with uniform dimensional change. It may change dimension in a non-uniform manner, e.g., warp, in which case the interface between the heatsink and the device may change dimensions in a complicated manner. These changes may be small or large, and the interface must be able to provide thermal contact successfully despite these "gap changes". Since typical operation of an electronic device involves use at high temperature for an extended period of time, an interface which conforms to the high-temperature interface configuration will successfully allow cooling of the component during use.
It is believed, therefore, that a heatsink interface which assures a "dry", separable (e.g., no grease or material residue cleaning required after separation, and no grease containment issues), thermal connection which is conformable (to accommodate variations in flatness of heatsink and/or device face), is convenient to assemble and apply to existing electronic devices and heatsinks, is of relatively low-cost, and provides relatively low thermal resistance would constitute a significant advancement in the art.